A general solar cell is a double-sided electrode type solar cell, which includes an electrode on both a light-receiving surface and a back surface. As a solar cell free from a shading loss caused by an electrode, a back contact solar cell has been developed (e.g. Patent Document 1). Since a back contact solar cell includes an electrode only on a back surface, the back contact solar cell is free from a shading loss caused by a metal electrode on a light-receiving surface, and is thus expected to exhibit high conversion efficiency.
A back contact solar cell includes a p-type semiconductor layer and an n-type semiconductor layer on the back side of a semiconductor substrate. FIG. 7 shows a back contact solar cell in which a p-type semiconductor layer and an n-type semiconductor layer are provided in a comb shape on the back side. In a solar cell 800, a p-type semiconductor layer 821 and an n-type semiconductor layer 822 which extend in a y direction are provided alternately along an x direction. At one end in the y direction, the n-type semiconductor layer is provided so as to extend in the x direction, and at the other end in the y direction, the p-type semiconductor layer is provided so as to extend in the x direction. Accordingly, a p-type semiconductor layer-formed region and an n-type semiconductor layer-formed region are provided in the shape of interdigitated comb teeth. With this structure, exited photocarriers in the semiconductor substrate by incident light from the light-receiving side can be efficiently collected in each conductive semiconductor layer.
An electrode for extracting collected carriers to outside is provided on each of the n-type semiconductor layer and the p-type semiconductor layer. Electrodes 841 and 842 extending in the y direction are each referred to as a finger electrode. Electrodes 846 and 847 extending in the x direction are each referred to as a bus bar electrode, and connect the end parts of a plurality of finger electrodes. Back contact solar cells in which a semiconductor layer and an electrode thereon are disposed in comb shape are connected in series, thus modularized. As shown by dashed lines in FIG. 7, wiring members 851 and 852 are mounted to bus bar electrodes 846 and 847, respectively, thereby an electrode on a p-type semiconductor layer of one solar cell and an electrode on an n-type semiconductor layer of an adjacent solar cell are connected through the wiring member.
An electrode having a comb shape structure as described above has a large carrier collection loss caused by series resistance because a distance K (carrier collection distance) between an end of a finger electrode and a bus bar electrode is substantially equal to a length Ly of one side of a semiconductor substrate. As the size of the substrate increases, the carrier collection distance increases, and therefore the loss tends to increase.
The series resistance can be reduced by increasing the cross-sectional area of the finger electrode. However, for spacing apart adjacent electrodes from each other, the width of the electrode in the x direction should be made smaller than the width of the semiconductor layer. When the width of the semiconductor layer is increased, the transport distance of photocarriers generated in the semiconductor substrate increases, and therefore a loss caused by carrier recombination increases. When the height of the electrode is increased, stress at the interface between the electrode and the semiconductor layer increases, leading to occurrence of warpage of the cell, and electrode delamination, and so on. Thus, there is a limit to reduction of series resistance by increasing the cross-sectional area of the finger electrode.
Patent Document 2 etc. suggests using a wiring sheet for connection of finger electrodes and connection of adjacent cells. For example, a solar cell 900 shown in FIG. 8 includes only finger electrodes 941 and 942 extending in a y direction, and does not include a bus bar electrode that couples the finger electrodes. A wiring sheet 950 includes finger electrode sections 951 and 952 substantially identical in shape to the finger electrodes 941 and 942 of the cell and bus bar electrode sections 956 and 957 connecting the finger electrodes to each other on a surface of a base material 960 which faces the cell. In modularization, a wiring sheet 950 is disposed on the cell 900, and the finger electrode of the cell and the finger electrode of the wiring sheet are connected to each other as shown in FIG. 9. Accordingly, carriers of all the finger electrodes can be collected through the bus bar sections of the wiring sheet. By increasing the heights of the finger electrodes 951 and 952 of the wiring sheet 950, series resistance can be reduced without increasing the heights of the finger electrodes 941 and 942 of the cell 900.